I first want to preface this article by saying I 100% was one of these people. I would wear the weight belt around the gym, doing curls and leg extensions and shoulder presses with it on. I would also wear a waist trainer (also known as a squeem or corset) while I did my cardio. They are big in the bodybuilding world and promoted as something to "keep your waist tighter." But, is that true? What do they actually do?
A weight belt is nothing more than an artificial brace. Bracing is a breathing technique where you breathe into your stomach, pushing out your transverse abdominal wall 360 degrees around your body. This provides stability throughout your midsection as you perform heavy movements - typically heavy compounds like squats or deadlifts. Eventually, you will reach a weight where your core cannot keep a solid brace and at this point, you can add assistance in the form of a weight belt. The problem is some people wear them ALL THE TIME. If bracing is a breathing and abdominal technique, artificially enhancing it teaches your body not to engage the proper core muscles when you don't have it on. Your core becomes weak. Weak cores are no bueno.
P.S. You can still have a 6-pack and have a weak core.
Every exercise you perform should be a core movement. I tell people all the time to keep their core tight while performing curls, rows, shoulder presses, etc. In the gym, your core should almost always be tight i.e. contracted while performing movements. It's indirect abdominal training and if you always have a weight belt on, you're missing out on a lot of benefits.
Now... waist trainers. Again, I used to wear one while doing cardio because I fell into the marketing bullshit that it keeps your waist tighter. As a bodybuilder, you want that; broad shoulders, TIGHT waist, and big legs. It is also a common thought that doing a lot of abdominal work will make your midsection look "blocky." Waist trainers can help keep your core tight by atrophying (destroying) your ab muscles. Think about it... if you break an arm and you're in a caste for however long when you remove that cast your arm is much smaller... it atrophyed, you lost muscle because you weren't using your arm. The same thing applies to waist trainers and your core. Sweet, you have a tighter waist, but you are literally destroying your core and you need your core for a lot of things in the gym and in life.
Furthermore, training your abs requires weight. Abs are a muscle too, they should be treated like a muscle. Will you gain some size in your abs? Of course, this is how you get the 3D abs that POP. Will it make your waist look "blocky?" No. If you're a competitor and you feel like your waist is blocky, you're not lean enough.
What's the conclusion: use a weight belt when you are lifting heavy weights and you can't keep your brace, other than that, stay off of it. Train your abs and waist trainers are silly.